Update comment and cleanup doc links

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

src/registers/segmentation.rs

@@ -6,13 +6,15 @@ use bit_field::BitField;
 use core::fmt;
 // imports for intra doc links
 #[cfg(doc)]
-use crate::registers::control::Cr4Flags;
+use crate::{
+    registers::control::Cr4Flags,
+    structures::gdt::{Descriptor, DescriptorFlags, GlobalDescriptorTable},
+};
 
 /// An x86 segment
 ///
 /// Segment registers on x86 are 16-bit [`SegmentSelector`]s, which index into
-/// the [`GlobalDescriptorTable`](crate::structures::gdt::GlobalDescriptorTable). The
-/// corresponding GDT entry is used to
+/// the [`GlobalDescriptorTable`]. The corresponding GDT entry is used to
 /// configure the segment itself. Note that most segmentation functionality is
 /// disabled in 64-bit mode. See the individual segments for more information.
 pub trait Segment {
@@ -107,20 +109,27 @@ impl fmt::Debug for SegmentSelector {
 
 /// Code Segment
 ///
-/// Most fields like the segment base and limit are unused in 64-bit mode.
-/// Only the LONG_MODE, PRESENT, USER_SEGMENT, EXECUTABLE, DEFAULT_SIZE (default operand size)
-/// and DPL_RING_3 (descriptor privilege-level) fields of the
-/// descriptor are recognized by the CPU. Some of them (e.g. DEFAULT_SIZE) are required to hold
-/// a specific value and can't be changed. Others like the DPL_RING_3 and LONG_MODE
-/// can be used to change privilege level or enable/disable long mode.
+/// While most fields in the Code-Segment [`Descriptor`] are unused in 64-bit
+/// long mode, some of them must be set to a specific value. The
+/// [`EXECUTABLE`](DescriptorFlags::EXECUTABLE),
+/// [`USER_SEGMENT`](DescriptorFlags::USER_SEGMENT), and
+/// [`LONG_MODE`](DescriptorFlags::LONG_MODE) bits must be set, while the
+/// [`DEFAULT_SIZE`](DescriptorFlags::DEFAULT_SIZE) bit must be unset.
+///
+/// The [`DPL_RING_3`](DescriptorFlags::DPL_RING_3) field can be used to change
+/// privilege level. The [`PRESENT`](DescriptorFlags::PRESENT) bit can be used
+/// to make a segment present or not present.
+///
+/// All other fields (like the segment base and limit) are ignored by the
+/// processor and setting them has no effect.
 #[derive(Debug)]
 pub struct CS;
 
 /// Stack Segment
 ///
 /// Entirely unused in 64-bit mode; setting the segment register does nothing.
 /// However, in ring 3, the SS register still has to point to a valid
-/// [`Descriptor`](crate::structures::gdt::Descriptor) (it cannot be zero). This
+/// [`Descriptor`] (it cannot be zero). This
 /// means a user-mode read/write segment descriptor must be present in the GDT.
 ///
 /// This register is also set by the `syscall`/`sysret` and